Powder coatings, which are dry, finely divided, free flowing, solid materials at room temperature, have gained considerable popularity in recent years over liquid coatings for a number of reasons. For one, powder coatings are user and environmentally friendly materials, since they are virtually free of harmful fugitive organic solvent carriers that are normally present in liquid coatings. Powder coatings, therefore, give off little, if any, volatile materials to the environment when cured. This eliminates the solvent emission problems associated with liquid coatings, such as air pollution and dangers to the health of workers employed in coating operations.
Powder coatings are also clean and convenient to use. They are applied in a clean manner over the substrate, since they are in dry, solid form. The powders are easily swept up in the event of a spill and do not require special cleaning and spill containment supplies, as do liquid coatings. Working hygiene is, thus, improved. No messy liquids are used that adhere to worker's clothes and to the coating equipment, which leads to increased machine downtime and clean up costs.
Powder coatings are essentially 100% recyclable. Over sprayed powders can be fully reclaimed and recombined with the powder feed. This provides very high coating efficiencies and also substantially reduces the amount of waste generated. Recycling of liquid coatings during application is not done, which leads to increased waste and hazardous waste disposal costs.
Despite their many advantages, powder coatings are generally not employed in coating heat sensitive substrates, such as wood and plastic. Heat sensitive substrates demand lower cure temperatures, preferably below 250.degree. F., to avoid significant substrate degradation and/or deformation. Lower cure temperatures are not possible with traditional heat curable powders. Unsuccessful attempts have been made to coat heat sensitive substrates with traditional powders.
For instance, when wood composites, e.g., particle board, fiber board, and other substrates that contain a significant amount of wood, are heated to the high cure temperatures required for traditional powders, the residual moisture and resinous binders present in the wood composites for substrate integrity are caused to invariably evolve from the substrate. Outgassing of the volatiles during curing results in severe blisters, craters, pinholes, and other surface defects in the hardened film finish. Furthermore, overheating causes the wood composites to become brittle, friable, charred, and otherwise worsened in physical and chemical properties. This is unacceptable from both a film quality and product quality viewpoint.
Low temperature UV curable powders have recently been proposed for coating heat sensitive substrates. UV powders still require exposure to heat, which is above either the glass transition temperature (T.sub.g) or melt temperature (T.sub.m), to sufficiently melt and flow out the powders into a continuous, smooth, molten film over the substrate prior to radiation curing. However, the heat load on the substrate is significantly reduced, since UV powders are formulated to melt and flow out at much lower temperatures than traditional powder coatings, typically on the order of about 200.degree. F. Therefore, UV powders only need to be exposed to enough low temperature heat required to flow out the powders into a smooth molten film.
Curing or hardening of UV powders is accomplished by exposing the molten film to light from a UV source, such as a mercury UV lamp, which rapidly cures the film. Since the crosslinking reactions are triggered with UV radiation rather than heat, this procedure allows the powder coatings to be cured more quickly and at much lower temperatures than traditional heat curable powders.
Another significant advantage of UV curable powders is that the heated flow out step is divorced from the UV cure step. This enables the UV powders to completely outgas substrate volatiles during flow out and produce exceptionally smooth films prior to the initiation of any curing reactions. Accordingly, the film finishes created with UV powders are known to have extraordinary smoothness.
One drawback is that opaque pigmentation of UV curable powders is known to be problematic. Opaque pigments inherently absorb, reflect, or otherwise interfere with the transmittance of UV light through the pigmented coating, and, consequently, impede the penetration of UV light into the lower layers of the pigmented film during curing. Pigmented UV powders, when cured, can still provide exceptionally smooth film finishes with good surface cure properties, including good solvent resistance. However, pigmented UV powders are not able to be adequately cured down through the film to the underlying substrate. As a result, pigmented UV powder coatings exhibit poor through cure properties, including poor pencil hardness, poor adhesion, and poor flexibility. Clear UV powder formulations which are applied as thicker films greater than about 2 mils present similar curing problems. Most UV curable powder coatings produced today are formulated as thin clear coats for wood and metals without pigmentation.
EP Publication No. 0 636 669 A2 to DSM, N.V. dated Feb. 1, 1995 discloses UV or electron beam radiation curable powder coatings which can be applied to heat sensitive substrates, such as wood, e.g., medium density fiber board, and plastic. The UV powders of EP 0 636 669 A2 contain: a) an unsaturated resin from the group of (semi)crystalline or amorphous unsaturated polyesters, unsaturated polyacrylates, and mixtures thereof, with unsaturated polyesters derived from maleic acid and fumaric acid being especially preferred; b) a crosslinking agent selected from an oligomer or polymer having vinyl ether, vinyl ester or (meth)acrylate functional groups, with vinyl ether functional oligomers being especially preferred, such as divinyl ether functionalized urethanes; and, c) a photoinitiator for UV or electron beam radiation cure, in which the the equivalent ratio of polymer unsaturation to crosslinker unsaturation is preferably 1:1. However, the UV powders of EP 0 636 669 A2 are practically limited to being formulated as unpigmented, i.e., clear, coatings, as demonstrated in Example 1. A similar clear coat UV powder based on an unsaturated polyester, an allyl ether ester crosslinker, and a hydroxyketone photoinitiator is disclosed in Example 2 of International Publication No. WO 93/19132 to DSM, N.V. dated Sep. 30, 1993.
K. M. Biller and B. MacFadden (Herberts Powder Coatings), UV-Curable Powders: A Marriage Of Compliant Coatings, Industrial Paint & Powder, pp. 22-25 (July 1996) and K. M. Biller and B. MacFadden (Herberts Powder Coatings) UV-Curable Powder Coatings: The Perfect Marriage of Compliant Coatings, Radtech North America 1996 Conference, pp.437-445 (Apr. 28-May 2, 1996), suggest the incorporation of special solid UV initiators which are designed to activate despite the presence of various pigmentations. Presumably, these special UV initiators have some ability to absorb UV light at wavelengths above the reflectance of the pigments.
F. M. Witte and E. S. de Jong (DSM, N.V.), Powder Coatings On Heat Sensitive Substrates, presented at the DSM, N.V. seminar held in Amsterdam (Mar. 27-28, 1996), disclose that some success has been achieved in the laboratory with pigmented UV curable powder coating formulations based on a binder which comprises a blend of: a) an unsaturated polyester resin, e.g., an unsaturated polyester derived from maleic acid; and, b) a vinyl ether functionalized polyurethane crosslinking agent. This binder is similar to that described in the aforementioned EP 0 636 669 A2. In these formulations, a special class of UV photoinitiators especially suited for pigmented UV coatings are used, e.g., bis-acylphosphineoxides and a 75/25 blend of a hydroxyketone (Irgacure 184) and bis-acylphosphineoxide, which is now available as Irgacure 1800 from Ciba-Geigy Corporation. Pigmented formulations with varying amounts of pigments between 5 and 20 wt. % are applied electrostatically to medium density fiber board at a layer thickness of 100 microns. Yet, although sufficient hiding is achieved with pigment loadings at 15 wt. % and 20 wt. % pigment, the pendulum hardness and, consequently, the through cure is not fully developed shortly after UV curing, which is undesirable.
The state of the powder coating art, therefore, is that opaquely pigmented or thick filmed UV curable powder coatings cannot be fully cured with UV light.
U.S. Pat. No. 4,753,817 to Meixner et al. discloses opaquely pigmented liquid UV radiation curable coatings for application to wood, wood-like materials and films of plastic. Such pigmented liquid UV paints are made by incorporating hydroperoxide thermal initiators alongside the UV photoinitiators in liquid resins derived from copolymerizable monomer-free air-drying unsaturated polyesters. The inclusion of the hydroperoxides in the liquid UV formulation is said to improve the cure at the lower layers of the coating that can not be penetrated by UV radiation due to the opacity of the pigments. The liquid paints also contain other essential ingredients not found in powder coatings, including plasticized colloidal cotton, e.g., nitrocellulose, siccatives to promote air-drying, and also volatile organic solvents to adjust the processing viscosity for liquid applications, e.g., toluene, xylene, isopropanol, and butyl acetate. These pigmented UV liquids are first applied to the substrate in liquid form, next pregelled at temperatures of 120.degree. F. to 175.degree. F., and then hardened under UV radiation. There is nothing in Meixner et al. that provides any indication that solid compositions could be made that are suitable as powder coatings.
In addition, the liquid UV coatings of Meixner et al. have a number of other shortcomings. For example, the UV liquids contain organic solvents, which generate physiologically and environmentally harmful solvent emissions during drying. As previously mentioned, powder coatings are essentially solventless and nonpolluting substances. The UV liquids also have pot lives only up to 5 hours, which is rather short. This would not be a suitable system for a powder coating which must not advance for several weeks at room temperature, especially in a reclaim powder coating booth. Also, cure for the Meixner et al. system occurs between 120 and 175.degree. F. At such temperatures, significant pregelation or even curing during extrusion would result with a powder coating, which, in turn, would cause processing difficulties as the powder would take a set in the extruder, leading to excessive extruder downtime and expensive clean out costs. The UV liquids are based on lower molecular weight polyester resins, which are highly reactive liquids at room temperature that cure without crosslinkers. Typical polyester powder coatings require resins with higher molecular weights or higher T.sub.g 's for the material to remain solid at room temperature, which tends to reduce the reactivity of the resin and require a crosslinker. Also, the UV liquids containing lower molecular weight species would be expected to transport more readily into biological tissues. If improperly handled, the highly reactive UV liquids could be transported through the skin of an individual and poisonously affect physiological functions. The use higher molecular weights species in powder coatings reduces handling problems and reduces the risk of invasion into biological tissues.
Moreover, the liquid UV coatings of Meixner et al. do not require an initial melt and flow out step in order to form a smooth film over the substrate, as required with UV powders, since these coatings are liquids at room temperature and inherently flow out as smooth films when poured. Therefore, incorporation of a thermally activated peroxide cure component in a liquid UV coating is not problematic from a film quality standpoint. However, the inclusion of a thermal peroxide cure component in a UV powder coating would be expected to produce coatings having poor flow out behavior and, consequently, poor film qualities. Conventional wisdom would lead one to expect that the addition of a thermal cure component to a UV curable powder would cause pregelation during the heated flow out step and cause undesirable surface roughness, such as orange peel or low gloss, in the hardened film finish, detrimentally affecting the film quality and aesthetic appearance of the coating. Blisters, craters, pinholes, and other surface defects would also be expected to be visually evident on the surface of the hardened film. Additionally, significant pregelation in the extruder during melt blending the powder ingredients would be expected when using a thermal initiator, causing processing difficulties as the powder blend would begin to cure and take a set in the extruder, which would lead to considerable extruder down time and increased clean out costs.
What is needed is an opaquely pigmented and/or thick filmed UV curable powder coating composition that is suitable for coating heat sensitive substrates, especially wood, wood composites, and plastic, and that can be fully cured through the incorporation of a thermal initiator alongside the usual UV initiator without detracting from the exceptional smoothness of the hardened film finish.